The purpose of this invention is to provide improved foam semiconductor polishing pads and belts by a novel method of mechanical frothing.
State-of-the-art semiconductor polishing pads are high density polyurethane foams that have a functional porous structure, which aids the distribution of polishing slurry and reduces hydroplaning, for example. The state-of-the art pads for semiconductor polishing are foamed by the inclusion of thin-walled, hollow plastic beads, which can potentially provide a controlled and consistent microcellular structure.
However, there are some limitations to the use of hollow microspheres in polishing pads. The hollow microspheres are typically limited in the sizes available, and they may be too abrasive for some delicate polishing operations, like certain steps in semiconductor manufacturing, including but not limited to chemical/mechanical polishing of soft metal layers. Typically the microspheres are extremely light weight and flammable, posing significant material handling difficulties, including dust explosion hazards. The light weight microspheres are difficult to disperse in the polyurethane resins, they tend to clump and foul process equipment, and they often entrain significant amounts of air, which leads to problematic variations in porosity of the cured foam. Also, the microspheres can distort, collapse, or melt if processed at certain high temperatures, that are routinely used in processing polyurethanes and other potential pad materials.
Although there are several different ways to create conventional high density polyurethane foam, including mechanical frothing and chemical blowing processes, pads produced by the conventional methods have not been successful in semiconductor polishing. In fact, lower technology pads for polishing glass have been produced by chemical blowing for many years, but they have not been as successful in semiconductor polishing, which is a more precise and more delicate application, because of the variability in pad cell structure and pad properties.
This invention comprises high density foam semiconductor polishing pads and belts with a controlled, reproducible micro-cellular structure that have been produced by a novel method of mechanical frothing. This invention provides an improved method for producing semiconductor polishing pads with consistent cell structure and properties, that perform equal to or better than the state-of-the-art polishing pads. The method also provides increased degrees of freedom and convenience in producing pads with different desired cell structures.
This invention comprises a method for making high density foam semiconductor polishing pads and belts with controlled, reproducible microcellular structure by mechanical frothing. The method involves agitating a liquid polymer resin at a controlled temperature and pressure in order to produce a stable froth. Next, the resin froth is metered under pressure to a mix head where it is typically combined with a desired amount of curative before being injected or poured into a mold. This invention also relates to the high density polishing pads and belts produced by the method.
The resin material is typically polyurethane but can be any suitable thermoset polymeric material. In the case of urethanes, any suitable formulation is acceptable, including the incorporation or utilization of various fillers, catalysts, additives, and surfactants. Catalysts and blowing agents can be used to create an open-celled structure in the polishing pad or to enlarge the cells after the mixture is poured into the mold. It has been found that nucleation surfactants, that are commonly used in the manufacture of low density blown foams, are surprisingly useful for producing a stable froth, which is critical to the present invention. One particularly useful nucleating surfactant is a block copolymer containing at least one block comprising polydimethylsiloxane and at least one other block comprising polyether, polyester, polyamide, or polycarbonate segments. The stable froth produced with the aid of the nucleation surfactants forms easily with simple agitation schemes and maintains its integrity when put through processing equipment with varying temperatures, pressures, and shear conditions.
Any suitable gas can be used as the frothing agent. Typically, dry nitrogen or dry air are used in the head space of the resin frothing vessel. Different cell sizes and different overall densities or porosities can be achieved by selecting the process temperatures, pressures, and agitation schemes. Different pressures can be used at different points or times in the process. For example, frothing, dispensing, and molding pressures can all be different. Preferably the stable froth is produced at a temperature of ambient to 100xc2x0 C. and at a pressure of ambient to 100 psig. Preferably the stable froth is metered to the mix head under a pressure of ambient to 200 psig.
Various mold or tooling designs may be employed to aid in maintaining or controlling the overall foam density and cell structure of the molded part.
Any suitable mixing, foaming, or dispensing equipment is acceptable, including those utilizing recirculation schemes.
An alternative variation of the present invention involves preparing the stable froth in continuous or semicontinuous fashion, in-line between a resin holding tank and the dispensing mix head.
The following non-limitative examples illustrate the invention: